In general, known spherical bearings of this type include bearings which are equipped with an inside member having a ball portion and an outside member enclosing and holding the ball portion of the inside member to be coupled to the inside member such that it can swing or rotate relative to the inside member. The outside member must undetachably enclose and hold the ball portion against any load acting on the inside member. Therefore, with such a spherical bearing, there remains a problem of what kind of structure to use for enclosing the ball portion in the outside member and for maintaining free swinging and rotary motions of the inside member and the outside member.
One known structure conventionally used for a spherical bearing is provided by preparing a metal casing as the outside member having a recess greater than the diameter of a ball portion and press-fitting the ball portion constituting the inside member enclosed in a self-lubricant resin sheet into the casing (JP A-57-79320, JP 63-188230 U, JP-A-05-26225, JP-A -07-190066, etc.). In this spherical bearing, since the resin sheet enclosing a ball portion is pressed between the ball portion and the casing to be subjected to elastic deformation, any gap between the ball portion and the resin sheet is eliminated to allow the ball portion to rotate in the casing without rattling. Further, since the ball portion is in slide contact with the resin sheet alone, there is no possibility of troubles such as biased wear of the ball portion even when the spherical bearing is used for a long time.
However, an outside member of this type, in which a resin sheet is sandwiched between a ball portion and a casing, has problems including difficulty in achieving smooth and light movement of a link mechanism that is configured using the spherical bearing because the resin sheet that is in contact with the ball portion in a compressed state makes the movement of the ball portion somewhat heavy. Another problem arises in that the resin sheet is likely to wear when it is used for a certain period of time because the resin sheet is in contact with the spherical surface of the ball portion under a pressure and in that the process of such wear is likely to cause rattling between the outside member and the ball portion. Further, still another problem arises in that the ball portion is likely to come off the outside member when a heavy load is applied to the spherical bearing because the resin sheet is elastically deformed under such a heavy load.
On the other hand, another structure for a spherical bearing is known in which an outside member is cast using a ball portion as a core to directly enclose the ball portion in the outside member (JP-A-48-019940). In this spherical bearing, the ball portion is first covered with a resin liner (with a thickness of approximately 0.5 mm) of low friction coefficient formed of fluororesin or the like, and is placed in the mold together with the resin liner, before the outside member is formed by die-casting of a zinc alloy or an aluminum alloy, the cast outside member enclosing and holding the ball portion through the intermediation of the resin liner. In this construction, it is possible to seal the ball portion in the outside member, with the gaps among the ball portion, the resin liner, and the outside member being completely eliminated; further, by selecting a self-lubricating material for the resin liner, it is advantageously possible to use the spherical bearing under no oiling condition.
However, when the outside member is thus die-cast using the ball portion covered with the resin liner as the core, the outside member after casting develops casting contraction, and tightens the ball portion through the resin liner. Thus, it has been impossible to freely rotate the ball portion relative to the outside member and the resin liner solely by casting the outside member.
In view of this, the spherical bearing as disclosed in JP-A-48-19940, after the die-casting of the outside member, an external force is applied to the outside member or the ball portion to cause the outside member to undergo plastic deformation, whereby a minute gap is formed between the ball portion and the resin liner, thereby securing free rotation of the ball portion.
However, to form a gap of an appropriate size between the ball portion and the resin liner, it is rather difficult to adjust the external force to be applied to the outside member or the ball portion. That is, when the external force is too small, a sufficient gap cannot be formed, and the ball portion and the outside member remain in close contact with each other, resulting in the movement of the ball portion relative to the outside member being rather heavy; on the other hand, when the external force is excessively large, the gap becomes too large, resulting in the ball portion rattling relative to the outside member. Further, even a slight rattling between the ball portion and the resin liner results in an increase in the gap between the ball portion and the resin liner due to a long-term use; thus, when, for example, the bearing is used in a link mechanism, it will be impossible to effect accurate transmission of motion or force between the inside member and the outside member.
Further, in a spherical bearing of this type, in order to prevent inadvertent swinging motion of the inside member relative to the outside member due to the action of slight vibration or the like, it would be convenient if it were possible to adjust to some extent the lightness of movement of the inside member with respect to the outside member, that is, the pre-load, according to use. However, in the method in which a gap is formed between the resin liner and the ball portion through plastic deformation of the outside member, it is difficult to effect fine adjustment on the size of this gap, which means that it is difficult to intentionally adjust the force with which the resin liner tightens the ball portion, that is, the pre-load.